VPE 511: Environment and Environmental Hygiene (2+1)

Introduction

MICROBIOLOGICAL EVALUATION OF WATER

• An understanding of the significance of the results obtained by the bacteriological examination of water supplies presupposes a knowledge of the possible bacterial forms which may be found in water. These forms may be broadly classified into two main groups.
• Firstly, saprophytic bacteria which are indigenous to water, are adapted to live and multiply at temperatures lower than body heat, e.g. at 20-22⁰C., and derive their nutriment from decaying organic matter. These free-living forms are of little importance from the hygienic standpoint, except that their presence in a water in very large numbers denotes an abundance of organic matter.
• The second broad group , which may be called the adventitious water micro-organisms, are introduced from outside sources and are incapable of surviving in water for an indefinite period.
• Amongst these are included bacteria precipitated from the air by rain or snow, soil bacteria washed in after heavy rainfall, and excretal bacteria from human or animal sources. It is with the two last named that the microbiological evaluation of water supplies is almost entirely concerned.
• It has already been mentioned that the present bacteriological techniques used for the detection of sewage pollution of water supplies are directed to the demonstration of known excretal bacteria, especially organisms of the coliform group, and , to a lesser extent, faecal streptococci and Clostridium welchii.
• The reason for this greater concentration on the coliform bacilli is partly because of the greater ease with which they can be demonstrated by cultural methods, and partly because Cl.welchii has been found to survive considerably longer in water than members of the coliform group.
• The presence therefore of this organism alone in a water sample may indicate a pollution more distant in time than is the case with members of the coliform group.